Electrostatic coating method and apparatus



Dec. 15, 1953 R. E. HELMUTH ELECTROSTATIC COATING METHOD AND APPARATUSFiled June 16, 1948 Fig.l

INVENTOR.

RICHARD E. H ELMUTH HWXQMW v Patented Dec. 15, 1953 ELECTROSTATICCOATING METHOD AND APPARATUS Richard E. Helmuth, Carmel, Ind., assignorto Ransburg Electro-Coating Corp., Indianapolis, Ind., a corporation ofIndiana Application June 16, 1948, Serial No. 33,371

6 Claims. (01. 117-93) 1 This invention relates to coating articles and.more particularly to improvements in an electrostatic method andapparatus for applying coatings to the exterior surfaces of hollowarticles of dielectric or nonconductive material such as,

past an extended backing electrode has been provided for the interior ofeach article being coated. U. S. Patent No. 2,247,963, issued tol-I. P.Ransburg and H. J. Green on July 1, 1941, illustrates one form of suchextended backing electrode.

It is apparent that the necessity for providing a backing electrodefitting the interior surface of the article surfaces of such article orfor electrical conformation, requires the fabrication of especiallyshaped backing electrodes for each form of article which it is desiredto coat. In cern tain instances, as where the opening leading into theinterior surface of the article is restricted as, for example, the neckof a'bottle, such bottleneck renders the use of the processimpracticable because of the difficulty of introducingthe backingelectrode into the interior of the article.

In certain instances, even though the entrance opening of a particulararticle is of a size which would allow free and easy mounting or removalof the article 'over or from the backing electrode, such type ofelectrode frequently cannot be used because the highly irregular surfacecontour of the interior of the article renders it impracticable, if notimpossible, to obtain the proper relationship between the article andthe backing electrode.

An obg ect of the present invention is to provide a method and apparatusfor electrostatically coating the exterior surfaces of hollow articlesof dieletric material in such manner that the operations be simply andeffectively carried out without necessitating the employment of 7 anybacking electrodes, especially shaped with respect to the articles to becoated and irrespective of the widely varying sizes and shapes of thearticles or their entrance openings or the degree of irregularitypossessed by their interior surfaces.

For a better understanding of the invention, together with other andfurther objects thereof, reference is had to the following specificationtaken in connection with the accompanying drawing, and the scope of theinvention will be pointed out in the appended claims.

In the drawing:

Fig. 1 is an end elevation of an electrostatic coating apparatus of thegeneral character with which my invention may be employed, andillustrates a typical use of the invention in the coating of glassbottles.

Fig. 2 is a side elevation thereof,

Fig. 3 is a detailed sectional view illustrating the use of theinvention in the application of coating to a glass tumbler or the like,and

' Fig. 4 is a detailed perspective view illustrating the use of theinvention in conjunctionwith the coating of a plastic radio cabinet orthe like.

The apparatus illustrated in Fig. 1 includes a conveyor 16 having spacedupstanding'spindles ll upon which articles are mounted. The'conveyor I0is arranged to travel between oppositely disposed, spaced electrodes 12,which are suitably supported by insulators l3 suitably affixed to aconventional coating booth (not shown). Electrodes l2 generally compriserectangular frames across which are strung in vertical spaced relation aseries of fine ionizing or discharge wires l4. Conveyor ID and spindlesH are connected through a conductor [5 to ground, whereas electrodes l2and Wires M are connected through a conductor 16 to one terminal of ahigh voltage source ll, the opposite terminal of which is connected by aconductor l8 to ground. It will be understood,of course, that ifdesired, the oppo- .site terminal of the voltage source may be conllandthe grounded connection thereby dispensed with, although generallythe ground type .of connection is preferred. Preferably, voltage necteddirectly. to the conveyor it and spindle source I! is adapted to supplydirect current with a voltage of the order of 1G0,000 volts, and theelectrodes are arranged so that the distance be conductive material aremounted upon and electrically connected to the spindles H, as such anarticle approaches the electrodes l2, an electrostatic field is createdbetween the ionizing wires 14 and the surface of such article, thearticle becoming during its movement past the electrodes l2 acomplementary or collecting electrode in the system.

One or more spray guns l9 are suitably supported beyond the electrodes(see Fig. 2) and are directed so as to introduce the coating materialfrom a supply source (not shown) in a direction generally parallel tothe path of travel of the articles into the space between the articlesand the electrodes. In this manner, the finely divided particles issuingfrom the spray gun become charged by the wires l4, and are thusattracted to the surfaces of the oppositely charged article.

Heretofore, when hollow:- articlesof .noncon' ductive material such as,.for-example. ceramic ware, glass or plastic were to be coated, it hasbeen the practice to back the interior. surface of the article with anextended metallic, form-fitting electrode supported upon and beingelectrically connected with the spindle ll. Such method andconstruction. is satisfactory in certain instances, as where the openingleading into the interior of the article to be coated is of. such sizeand shape with respect. to its inner surface as will permit the articleto be placed upon and removed from the conforming electrode. However,even in such cases, it is noted that variations in size and shape of theinterior surfaces of varying articles call for correspondingly varyingconforming plate electrodes. This naturally requires the maintenance ofa large inventory of such electrodes'in order to adapt the process andapparatus for use inconnection with articles of varying size and shape.Moreover, with certain types of articles, such as, for example, bottlesand the like, which generally have a narrow neck opening leading into"their interior surfaces, the prior art type of backing electrode cannotbe used because-the restricted opening prevents the mounting of suchtypes of. articles thereover.

By my .inventionJ. have overcome such .diiiiculties and have provided. amethod. and. apparatus for coating hollow articles which convenientlylends itself to applying. a uniformapplication of the coating materialto such articles without requiring the use of especially shapedelectrodes. I have discovered that if an. article with an opening whichleads into its interior is placed at or adjacent an isolated, sharpionizing point or points or a fine wire electrically connected with thespindle H, an ionizing zone or zone of corona discharge is createdwithin the interior of the article. Such discharge, in effect,establishes an intimate electrical contact with the entire interiorsurface of the article by virtue of which positive charges" (assumingthat the pointed electrode is charged positively) are distributed toevery point on such surface. A possible explanation or theory of theoperation in efiecting a uniform coating is that the nonconductive ordielectric character ofrthe article with ionizing zones or zones ofcorona discharge of opposite polarity located in spaced relationrespectively, internally and exteriorly thereof,

causes a polarization or electrical stress to take place in thenonconductivematerial. sothatl'the negatively charged particles ofcoating material are evenly and thoroughly attracted to and. depositedupon the positively charged exterior surface of the article.

In explaining the theory of the operation of the apparatus and method, Ido not wish to be confined to such theory in claiming my invention.

Referring again to Fig. 1 wherein is illustrated 4 the application ofcoating material to the exterior surface of a glass bottle 20, thespindle H is formed with an enlarged portion 2| which provides ashoulder 22 upon which the end of the inverted bottle 23 is seated.Extending upwardly and centrally of the enlarged portion 2! is aprojection. 24" which terminates in a sharp ionizing. pointv 25.Projection 24 may be formed integrally with the spindle H or mayconstitute a fine ionizing wire suitably afiixed on the spindle.Preferably, the pointed end 25 of the projection 24 terminates slightlywithin the neck of the bottle. It has been found by experimentatinn,howeventhat if the point extends into bottle for a substantial portionbeyond the bottle neck such as, for example, as indicated by dottedlines at 2B, the application of the coating material will be moreheavily applied to the exterior surfaces of the bottle lying in a planeabove the point.

Referring to Fig, 3, wherein is illustrated the application. of theimproved process to the coating of a glass. tumbler. 2'! or. the likethe spindle 3H is formed. witha flange or shoulder 3l'2 upon which restsa disk 3|3, preferably formed of insulatingv material. The upper end ofthe spindle projects centrally through an opening 3 provided in disk 31%and terminates in a sharpened point 3'15. The diameter of disk 3l3 issuch that the edge of the inverted tumbler 21 .is supported thereby.

Referring. to Fig. 4 wherein is shown a modification for adapting theprocess and. apparatus to the use of applyingv coatings to radiocabinets or the lil e,.the spindle 4| I has affixed thereto adjacent itspointed end 4l2 a spider M3, preferably of insulating material, theoutwardly projecting arms of which are of suflicient length to engage.and. support the edges of the radio cabi net 4 [4. Such radio cabinetsare commonly made of electrically insulating or nonconducting ma-.fifilials.

In allof. the illustrations, it is noted that the terminal point 25, 3150r 4 l 2 projects only slightly intov the interior of the hollowarticle. It has been found. in practice that the point may be sopositioned that it is coplanar with or extends slightly below the planeof the edges defining the. opening in the article, and that satisfactorycoating results may be achieved when the ionizing point is sopositioned.

The point or wire may even be formed so as to come .in contact with theinterior surface of the article when the article is mounted upon thespindle, the important requirement being that the point or wire becapable of creating an ionizingv zone or zone of corona discharge withinthe interior of the article. Preferably, in carrying out the operations,the spindles ll, 3 or 4H are arranged to. be. rotated so as to impartrotation to the articles during their traverse of the coating. zone.Such rotating mechanism may take the form of a. pulley such as 35 (Fig.l) aflixed to the spindles and arranged to be engaged by a continuouslydriven belt 3| mounted over driving pulleys such as 32.

While the invention is herein shown and described in connection with anupstanding spindle type of conveyor, it is to be understood, of course,that it is equally adaptable for use in conjunction with any type ofarticle support.

It is noted that by using such a method and apparatus as hereindescribed, articles of nonconductive material and of varying sizes andshapes. may be readily and easily accommodated without particular regardto the sizes of their entrance openings to their interior shape or tothe degree of irregularity possessed by their interior surfaceformations.

The requirement for especially shaped, extended backing electrodes iscompletely eliminated. In practicing the invention, the coating materialis issued from the spray guns such as 19, in exactly the same manner asdescribed in the foregoing. The establishment of an ionizing or coronadischarge zone within the interior of the nonconductive article, whichzone is of opposite polarity to the ionizing zones created by wires isin spaced relation from the exterior surface of the article, iseffective to establish an electrical field between the electrodes. Thefinely divided material issuing from the guns i9 is deposited upon theexterior surface of the article by the precipitating action of suchfield.

It has been found in practice that a coating of satisfactory thicknessand having a very high degree of uniformity can be obtained simply andefiiciently by the use of this method and apparatus.

What I claim is:

1. Apparatus for electrostatically coating the exterior surface of ahollow article of non-conductive material having an opening leading intoits interior, said apparatus comprising an ionizing discharge electrode,means for supporting the hollow article in spaced relation to saidionizing discharge electrode, said means being constructed and arrangedrelative to said ionizing discharge electrode whereby said ionizingdischarge electrode is at the opening of the hollow article and withinthe lateral confines thereof, a second ionizing discharge electrodesupported in spaced relation to the exterior surface of the article,means for establishing an electrostatic field between said electrodes ofsuflicient magnitude to create an ionizing zone about each, and meansfor introducing a spray of particled coating material into the fieldbetween the exterior surface of the article and said second dischargeelectrode for electrostatic deposition on the 1 article.

2. An apparatus according to claim 1 wherein said first mentionedionizing discharge electrode and the support for said article are formedintegrally.

3. A method of coating an exterior surface of a hollow article ofnon-conductive material which comprises the steps of creating in spacedrelation from and substantially within the boundaries of the interiorsurface of said article a localized zone of corona discharge ofpredetermined polarity to establish internally of the article andsubstantially throughout its confines a cloud of charged atmosphericparticles, simultaneously creating in spaced relation from the exteriorsurface of the article an ionizing zone of corona discharge of oppositepolarity to that of the atmospheric particles within said article,introducing a gaseous dispersion of particled coating material into theionizing zone exterior of said article to form a cloud of coatingmaterial particles charged oppositely to the internal atmosphericparticles, and establishing across the nonconducting walls of thearticle an electric field whereby the internal atmospheric particleswill be attracted to the internal surfaces of the Wall and the externalcoating material particles will be attracted to the external surfaces ofthe wall.

4. Apparatus for electrostatically coating the exterior surfaces of aseries of hollow articles of nonconducting material, each of saidarticles having an opening leading into its interior, said apparatus,comprising a conveyor, a first discharge electrode spaced from saidconveyor, a series of support members, each being carried by saidconveyor and being adapted to support one of the hollow articles, aseries of sharply pointed discharge electrodes, means for establishingan electrostatic field between said first discharge electrode and saidpointed electrodes of suficient strength to create an ionizing zone inthe vicinity of each electrode, each of said pointed electrodes beingdisposed at an articleopening and in such position that atmosphericparticles charged by it and attracted by said first electrode will beintercepted by the interior surface of the associated hollow article,and means for spraying coating material into the field between saidfirst electrode and the articles for electrostatic deposition on theexterior surfaces of the articles.

5. A method of coating the surface of a hollow article of nonconductingmaterial which comprises establishing between an ionizing electrodewithin the article and an ionizing electrode outside the article anelectrostatic field of sufficient strength to establish internally ofthe article and substantially throughout its confines a cloud of chargedatmospheric particles and externally of the article a surroundingionized atmosphere of a polarity opposite to that of the internal cloud,and introducing a gaseous dispersion of particled coating material intosaid surrounding ionized atmosphere and into the field exterior of thearticle for electrostatic deposition on the article under the action ofsaid field.

6. Apparatus for electrostatically coating the exterior surface of ahollow article of non-conducting material having an opening leading intoits interior, a first dischargeelectrode spaced from said support, asharply pointed discharge electrode, means for establishing between saidtwo electrodes an electrostatic field of sufiicient strength to createan ionizing zone in the vicinity of each electrode, said pointedelectrode being disposed in alignment with the opening of an article onsaid support and sufficiently close to such article that atmosphericparticles charged by it and attracted by said first electrode willimpinge upon the interior surface of the article on the support tocreate a surface charge, and means for spraying coating material intothe field between said first electrode and an article on the support forelectrostatic deposition on the exterior surface of such article.

RICHARD E. HELMUTH.

References Cited in the file ofthis patent UNITED STATES PATENTS NumberName Date 2,152,077 Meston Mar. 28, 1939 2,224,391 Huebner Dec. 10, 19402,287,837 Smyser June 30, 1942 2,334,648 Ransburg Nov. 16, 1943

3. A METHOD OF COATING AN EXTERIOR SURFACE OF A HOLLOW ARTICLE OFNON-CONDUCTIVE MATERIAL WHICH COMPRISES THE STEPS OF CREATING IN SPACEDRELATION FROM AND SUBSTANTIALLY WITHIN THE BOUNDARIES OF THE INTERIORSURFACE OF SAID ARTICLE A LOCALIZED ZONE OF CORONA DISCHARGE OFPREDETERMINED POLORITY TO ESTABLISH INTERNALLY OF THE ARTICLE ANDSUBSTANTIALLY THROUGHOUT ITS CONFINES A CLOUD OF CHARGE ATMOSPHERICPARTICLES, SIMULTANEOUSLY CREATING IN SPACED RELATION FROM THE EXTERIORSURFACE OF THE ARTICLE AN IONIZING ZONE OF CORONA DISCHARGE OF OPPOSITEPOLOARITY TO THAT OF THE ATMOSPHERIC PARTICLES WITHIN SAID ARTICLE,INTRODUCING A GASEOUS DISPERSION OF PARTICLED COATING MATERIAL INTO THEIONIZING ZONE EXTERIOR OF SAID ARTICLE TO FORM A CLOUD OF COATINGMATERIAL PARTICLES CHARGED OPPOSITELY TO THE INTERNAL ATMOSPHERICPARTICLES, AND ESTABLISHING ACROSS THE NONCONDUCTING WALLS OF THEARTICLE AN ELECTRIC FIELD WHEREBY THE INTERNAL ATMOSPHERIC PARTICLESWILL BE ATTRACTED TO THE INTERNAL SURFACES OF THE WALL AND THE EXTERNALCOATING MATERIAL PARTICLES WILL BE ATTRACTED TO THE EXTERNAL SURFACES OFTHE WALL.